# Design a Notification System — System Design

Source: https://www.geekswithgeeks.com/en/system-design/sd-design-notifications

> Fan-out to push, email, and SMS reliably at scale.

## Requirements

Many internal services need to notify users (order shipped, comment reply, price drop) via push, email, or SMS, respecting user preferences and avoiding spamming the same event twice.

## Decoupled with a queue

Producing services publish a `notification requested` event to a queue rather than calling providers directly. A **notification service** consumes it, checks user preferences and quiet hours, picks the channel, and calls the right third-party provider (APNs, FCM, SendGrid, Twilio).

## Idempotency and retries

A retried send must not double-notify the user. Dedupe by a stable notification ID before dispatching.

```text
on notification_requested(event):
  if seen(event.notification_id): return  # dedupe
  channel = pick_channel(user.prefs)
  provider[channel].send(event)
  mark_seen(event.notification_id)
```

Output:

```
Provider outages are isolated per channel with their own retry/backoff
```

## Batch to avoid spam

For chatty sources (many comments in a minute), batch related events into one digest notification within a short window instead of firing one push per event.